The invention relates to a method of producing mirrors, and to coated glass substrates incorporating highly reflecting "mirror" coatings.
The light reflecting properties of mirrors are generally provided by a layer of highly reflecting metal, especially silver, aluminium or chromium, applied to a glass or plastics substrate; copper layers are sometimes used as an alternative, but are generally less acceptable because of the strong red tint of the reflected light.
Silver coatings are generally applied to preformed glass plates, in the cold, by wet chemical methods in which a solution of silver salt is applied to the glass surface and reacted with a reducing agent which reduces silver ions present to silver metal which deposits on the glass surface. The silver used is not very durable in use and in practice requires protection by other layers, and these methods are generally unsuitable for application to glass on the production line on which it is formed so that a separate "silvering" line is required to produce the silvered glass.
Aluminium coatings are difficult to apply by chemical methods because of the strongly reducing nature of aluminium metal, and aluminium mirrors are generally produced by deposition methods carried out at low pressure e.g. by sputtering. Such low pressure methods are essentially batch processes and, like the wet chemical methods used for deposition of silver mirrors, are generally unsuitable for on-line application on the production line on which the glass is made.
U.S. Pat. No. 3,656,926 and U.S. Pat. No. 3,681,042 each propose a process for the production of mirror coatings by condensing a metal vapour on the hot glass surface on the production line on which the glass is made. According to U.S. Pat. No. 3,656,926, a body of molten metal, for example silver, aluminium copper or gold, is located adjacent the upper surface of the glass by a beam of refractory material extending transversely across the glass width, and an electrical current is passed through the beam of refractory material extending transversely across the glass width, and an electrical current is passed through the beam to heat the beam, for example to a temperature of 2000.degree. C., and evaporate the metal for condensation on the adjacent glass. According to U.S. Pat. No. 3,681,042 a body of molten metal, for example silver, aluminium, gold, copper or tin, is contained in a trough in a duct supported over the glass ribbon and heated to a high temperature, for example 2000.degree. C., and a carrier gas passed over the molten metal body in the trough to entrain metal vapour evaporating therefrom. The carrier gas containing the metal vapour is directed towards the glass surface where the metal vapour condenses on the glass. To improve the adhesion of the reflecting metal to the glass, it is further proposed to first deposit, in a similar manner, a keying layer of tungsten palladium, nickel or a palladium nickel alloy on the glass before deposition of the reflecting layer. Both these techniques require use of extremely high temperatures and neither has found commercial application.
Silicon has been deposited on hot glass during the glass production process to produce reflecting layers (which, like silver and aluminium layers, are substantially neutral in reflection colour) for use on architectural glazing for aesthetic and solar control purposes. GB-A-1507465, GB-A-1507996 and GB-A-1573154 relate to a continuous chemical vapour deposition method for producing float glass having such a silicon layer, and U.S. Pat. No. 4,661,381 describes a development of that method. However, such silicon layers do not provide the high reflections commonly required in mirrors, Thus REFLECTAFLOAT (trade mark) glass, commercially available from Pilkington Glass Limited of St Helens, England, has a reflection of about 50%, and MIRROPANE EP (trade mark) commercially available from Libbey-Owens-Ford Co, has a reflection of about 60%.
EP-A-0583871 discloses mirrors and their production in which a mirror coating comprises a stack of layers. The mirrors may be produced on-line during manufacture of the glass, for example during the float glass production process. The layers comprise materials which can be deposited on-line, for example non-metallic materials such as silicon, silicon dioxide, titanium dioxide, etc. It is disclosed that reflective metals, for example aluminium, chromium, cobalt or titanium may be used as an alternative to silicon and that a metal may be deposited by condensation of a metal vapor or by chemical vapor deposition using a suitable organometallic vapour. However, no specific conditions for metal deposition onto the glass substrate are disclosed.
GB-A-2248853 disclose a pyrolytic metal of coating glass with aluminium to form a mirror. A solution of an alane amine adduct is formed and the liquid is deposited onto heated glass. The adduct decomposes to form an aluminium coating. It is envisaged that the invention may be used in conjunction with float glass production, and suggested that the aluminium deposition may be carried out on hot glass, typically at 180.degree. C., emerging from the float glass process. Unfortunately, it has been found that coatings produced in the manner described are insufficiently durable for commercial application as mirrors.